1. Field of the Invention
The present invention relates to a superconducting coil device, and more particularly to a superconducting coil device which makes it possible to prevent damages to the thyristor that is provided for protecting the superconducting coil and/or the persistent current switch.
2. Description of the Prior Art
A superconducting coil device consists usually of a superconducting coil and a persistent current switch which short-circuits the superconducting coil that are accommodated in a cryogenic refrigerant represented by liquid helium.
Now, if the superconducting coil or the persistent current switch makes a transition for some reason to the resistive state while a superconducting coil device constructed as above is being operated in the persistent current mode, then the coil and the switch are in fear of being damaged by burning. For this reason, there is required some kind of protective system to cope with this kind of transition to the resistive state, that is, quenching. Of the various protective systems that can be thought of there has been proposed recently a system of protection that connects a diode to both ends of the superconducting coil, as shown by FIG. 1. According to this system, when a persistent current switch 14 is quenched while the device is being operated in the persistent current mode by cutting off the circuit 10 from a power supply 12, there is generated an inductive voltage between both ends of a superconducting coil 16. The voltage generated is directed in the forward direction of a diode 18 so that a current flows into the diode 18 to consume the energy accumulated in the superconducting coil 16, protecting the persistent current switch 14 and the superconducting coil 16. During operation of the device in the persistent current mode, the power leads 20 are separated for the purpose of suppressing heat flow into the cryostat, and the lead terminals 22 too are placed in a cryostat 24. Accordingly, the diode 18 is placed in the region of liquid helium or gaseous helium, namely, the cryostat 24.
When the persistent current switch 14 is quenched, the voltage between both ends of the superconducting coil 16, when the diode 18 begins to protect, is determined uniquely by the voltage-current characteristics of the diode 18. For a diode of 150 A class, current begins to flow at the voltage of the order of 10 V at 4.2K. In the case when the superconductor of the persistent current switch 14 is a perfectly stabilized conductor, it can happen that a part of the conductor for the persistent current switch 14 is quenched but the conductor reverts to the superconducting state. Even for such a case current sometimes flows into the diode 18. In the other hand, if several diodes 18 are connnected in series in order to prevent such malfunctioning of the diode 18, circuit protection may become impossible because of a large protection voltage that is applied continuously between the ends of the persistent current switch 14.